The invention relates to a direct injection type internal combustion gasoline engine and particularly to improvements in direct injection type internal combustion engines which employ both homogeneous charge combustion and stratified charge combustion.
A conventional engine injects gasoline into the air intake port upstream of the combustion chamber. The air-fuel mixture is then transported to the combustion chamber where it is burned. In contrast to this arrangement, a direct injection gasoline engine has the fuel injector located inside the combustion chamber so that the fuel is injected directly into the cylinder.
The combustion process of a conventional gasoline engine is limited to homogeneous charge combustion. A direct injection gasoline engine, on the other hand, selectively uses both stratified charge combustion and homogeneous charge combustion.
With stratified charge combustion, the fuel is injected into the combustion chamber during the compression stroke. The objective is to position a readily ignitable mixture in the vicinity of the spark plug while forming a surrounding air layer that contains little fuel. This process thus accomplishes stable combustion of an overall lean mixture. Lean combustion is accomplished in the stratified charge combustion process to improve fuel economy.
With homogeneous charge combustion, on the other hand, the fuel is injected into the combustion chamber in the induction stroke. Similar to a conventional gasoline engine, the mixture is then uniformly mixed to a stoichiometric ratio. This results in the generation of greater power under high-load operation.
In short, a direct injection gasoline engine employs stratified charge combustion at low load for improved fuel economy and homogeneous charge combustion at high load for greater power. This selective use of the two combustion processes achieves both low fuel consumption and high power output.
A variety of direct injection type internal combustion engines have been proposed.
One of two different physical phenomenons may be employed during stratified charge combustion. These phenomenons differ with respect to how air flows in the cylinder during stratified charge combustion. One type is "tumble" flow wherein a vortex is created in a vertical plane of the cylinder. Such flow is shown, for example, in U.S. Pat. No. 5,711,269 issued to Hideyuki Oda and others. The other type is "swirl" flow wherein a vortex is created in a horizontal plane of the cylinder during stratified charge combustion.
This invention employs swirl (that is, horizontal vortex) flow during stratified charge combustion.
A direct injection type internal combustion engine employing swirl flow during stratified charge combustion is described, for example, in U.S. Pat. No. 5,553,588 issued to Takeshi Gono and others. The internal combustion engine described in this patent is arranged as follows. A non-circular cavity combustion chamber which is eccentric relative to a piston outer peripheral circle is formed at the top section of the piston. A fuel injector valve is disposed to inject fuel toward the cavity combustion chamber near the upper dead center of the piston. The cavity combustion chamber is a reentrant type so as to confine fuel and swirl therein. In other words, the cross-sectional area of the combustion chamber is reduced toward the top of the piston. In order to produce strong swirl in this cavity combustion chamber, one of a pair of intake ports is arranged as a helical port, and an air control valve is provided to open or close the other intake port.
The internal combustion engine in this patent is arranged such that, during lean combustion, the above-mentioned air control valve is closed so that fresh air is introduced only through the one helical port to produce strong swirl inside the cylinder. Since this swirl is introduced into the cavity combustion chamber with ascent of the piston, a combustible air-fuel mixture is formed and carried to near the spark plug by injecting fuel into the cavity combustion chamber near the top dead center of compression. Accordingly, ignited combustion can be accomplished by making ignition at a suitable timing.
However, the inventors of this invention have recognized several shortcomings of this arrangement. In the above-mentioned arrangement of the piston, the cavity combustion chamber at the piston top section is non-circular, such as generally triangle-shaped or cocoon-shaped, and therefore a very strong swirl must be produced in the cylinder by using a helical port in order to create swirl having a sufficient intensity.
However, using such a helical port increases intake air resistance at high power output under the high load operation. To compensate for this reduced output, a variable valve timing system is provided, which complicates construction and increases cost.
Additionally, by forming the cavity combustion chamber in a reentrant shape, swirl and the air-fuel mixture is maintained inside the cavity combustion chamber during stratified charge combustion. However, fuel tends to stagnate in such a cavity combustion chamber during homogeneous charge combustion, thereby degrading performance. Also, the required depth of the cavity combustion chamber increases the weight of the piston, which in turn increases noise and vibration.